Filoviruses are filamentous, enveloped viruses with non-segmented, negative-sense RNA genomes associated with outbreaks of severe illness in humans. Reston ebolavirus (RESTV) is unique among the filoviruses in that, despite a number of documented human exposures to the virus, no human illness has been reported. The apparent avirulence of RESTV, which is in stark contrast to some other filoviruses such as Ebola virus, makes RESTV of particular interest. Despite sequence divergence, RESTV genetic organization is essentially the same as for other filoviruses and many of the functions of individual viral proteins are conserved, including replication and innate immune evasion functions. Significant approaches/questions that can be addressed with RESTV include (1) comparative in vivo studies to define significant differences in pathogenesis and immune studies between an attenuated (RESTV) and pathogenic (e.g. EBOV) filovirus. (2) Determination of the genetic stability of RESTV in vivo. (3) Characterization of proteins and RNA elements conserved between filoviruses. (4) Identification of virus-host interactions required for replication and which may contribute to virulence/attenuation. Points (3) and (4) can be addressed more safely with RESTV than with more deadly filoviruses. We propose a Developmental Project that builds on the established and productive collaboration between the Basler and Geisbert labs to develop tools and basic knowledge that will lay the groundwork for in depth research into RESTV biology. The project will define the basics of RESTV pathogenesis, so as to facilitate better understanding of its virulence and attenuation. The study will also build on our understanding of the filovirus VP35 protein as a critical innate immune antagonist to develop an attenuated but replication competent RESTV mutant that can be safely studied outside of the BSL4 laboratory.